1. Field of the Invention
The present invention relates to a method and an apparatus for copying transparent originals, or transparencies, to light-sensitive material, using an illuminator with controllable pixels which can be switched to light or dark for illuminating the image. The apparatus of the invention also includes an objective for imaging transparencies onto light-sensitive material.
Copies of photographic pictures which exhibit large differences in optical density in different areas, are frequently overexposed in the light areas or underexposed in the dark areas. Consequently, details or finer features that are reproduced on paper copies are either very poorly or hardly recognizable at all.
2. Description of the Related Art
A copying apparatus is described in German Patent DE PS 28 20 965 wherein the illuminator for the photographic film is a light source and a liquid crystal display (LCD). The display is electrically addressed so as to generate a black and white negative mask of the original. When the paper is exposed through both the mask and the original image, the optical density is compressed as required. To generate a corresponding black and white mask, the LCD must generate gray scale values. If an LCD with a ferro-electric liquid crystal is used, then the gray scale values to be produced have to be rastered, since the individual pixels of an LCD of this type can only be switched to either light or dark.
U.S. Pat. No. 5,638,153 describes to subdivide the LCD into fixed areas. These areas which can be called macro pixels, are subdivided into six individually addressable pixels having areas with a ratio of 1:2:4:8:16:32. By combining and addressing the pixels in different ways, macro pixels with 64 brightness levels can be generated.
It has been observed that a mask generated in this way can be used to copy finely detailed photographic images with excellent results, without causing Moire patterns, i.e. beat frequencies, which are typical in the reproduction of fine features. However, when images with large and very uniform areas are copied, the copy frequently exhibits features which are absent in the transparent original image.
It has also been observed that the masking process blurs sharp edges in the transparent original to such a degree that the paper copy exhibits non-uniform fringes at those edges.